This invention relates generally to prime mover systems and, more specifically, to power generation.
In most power generation applications, synchronous motors are driven to generate AC power. When so used, the frequency of AC power output is dependent upon the process used to apply torque to the synchronous motor. Constant torque at standardized values rarely exists in nature to precisely turn a synchronous machine. The rotational rate or angular velocity varies greatly. Because angular velocity is proportionate to the resulting frequency and voltage of AC power, where the angular velocity of the torque source is variable, the frequency and voltage of the resulting power is variable.
Randomly variable frequency AC power is not very useful. It is impossible to synchronize such power with a power supply network in a commercially practical manner. Such power cannot drive most applications designed for 60 Hz. line supply voltages. Few applications can tolerate such frequency variability.
To overcome the frequency variability in power generation, the solutions have been of two types: input and output solutions. Input solutions are mechanical and govern the power transfer to the synchronous machine. Output solutions condition electrical power garnered from the synchronous machine.
Traditionally, design constraints on power processing have required selection of components based upon the peak power to flow through those components rather than to design around the mean as in DC power systems. Periodically recurring peaks in the voltage and current waveforms for each phase develop recurrent power transfer well above the mean. Nonetheless, the power drawn from such a system as an aggregate is constant. For instance, the total power drawn from a balanced three-phase source by a balanced resistive load is constant. That is
Pt(t)=(V2/R) [sin2xcfx89t+sin2(xcfx89t+xc3x8)+sin2(xcfx89t+2xc3x8)]=1.5 (V2/R)xe2x80x83xe2x80x83(1)
where Pt=time value of power;
V=peak line voltage;
R=load resistance (per phase);
xcfx89=source frequency; and
xc3x8=2xcfx80/3.
This fact is exploited in a cycloconverter at discrete frequencies. Small fluctuations in frequency are passed to the output. This need not be the case. The power transferred from source to load is not a function of time. The transfer of power can be accomplished without storing energy within the processor between input voltage cycles.
The traditional input solution to electrical power generation in applications such as aircraft has been through the Constant Speed Drive (CSD) coupled to a generator providing, for example, 115 VAC with three-phase power at a constant 400 Hz. In more recent times this arrangement has combined the CSD and generator into an Integrated Drive Unit, or IDU. With a constant frequency power output this has been a creditable solution, albeit expensive to buy and to maintain.
More recently, the output solution has been the Variable Frequency (VF) and cycloconverter systems. Cheaper of these two options, VF presents a load with power such as 115 VAC, three-phase power but only has a distribution capability at a frequency proportional to the engine speed. For a turbofan engine, for instance, this is usually 2:1. However, because of the wide range of frequency variation, power conditioning would be essential for almost all cases and, when added to the procurement, the additional cost of attendant motor controllers becomes prohibitively expensive.
In recent years, power has been generated with a Variable Speed Constant Frequency (VSCF) cycloconverter. A cycloconverter is a power electronics device designed to provide a variable voltage, constant frequency AC drive in a one stage operation, suitable for supply to an AC motor. These devices work by generating very high frequency three-phase power then selectively drawing voltages from the peaks of the three phases in a manner to construct rough approximations of lower frequency waveforms.
While a VSCF system unit (cycloconverter) does have the ability to produce AC and DC simultaneously, it does not produce clean waveforms. Voltage regulation is accomplished by a series of magnetic amplifiers, transformers, and bridge rectifiers. The VSCF drive uses a simple drive system and lets the alternator produce an electrical supply that is not well-controlled, which is then shaped by a solid-state electrical unit. Nonetheless, the resulting waveform includes several harmonics that impart an imaginary component to the power and may interfere with the function of the load.
Still another means of generating constant frequency power from a variable source of torque is based upon converting and rectifying power to DC before inverting the DC power to AC power such as 60 Hz. line voltage. This approach requires a converter that can sink current of opposite polarity to the output voltage. However, most converters cannot accommodate a non-unity power factor load. Additionally, for a given power level, in a single-phase rectifier, the current pulses that make up a ripple may be four to five times as large as the peak of the current waveform for an equivalent unity power factor load. Such current requires much larger conductors to minimize resistive losses. In polyphase rectifiers, the current peaks are not as large since smaller peaks occur more frequently, but the power quality is not as good because of large current transitions.
Cycloconverters and devices for converting and inverting power use, such as transformer rectifier units that operate at line frequency, chop power at low frequencies, thereby creating low frequency fundamental sinusoids. As a result, large and heavy transformers and large capacitors are typically used to store energy for smoothing peaks and filling valleys in the waveform. Introduction of such elements often adds reactive factors that affect the power factor. In such configurations, the reactance causes the current to either xe2x80x9cleadxe2x80x9d the voltage or to xe2x80x9clagxe2x80x9d the voltage. Like the input solution, power conditioning is necessary for power-factor correction. Often this includes use of synchronous motors spinning in xe2x80x9cno loadxe2x80x9d states. All of these solutions prove to be costly. The need for constant frequency power has justified these solutions.
U.S. Pat. No. 6,466,468 issued to Douglas York on Oct. 15, 2002 presented a novel means of producing variable frequency power without rectifying that power to direct current. The teaching of the York patent, incorporated by this reference, is to chop power to a frequency significantly above the range of frequencies used to drive the motor for the application and, by phase shifting the chopped power, producing a power wave form at a desired frequency that represents a product of the input power at the input frequency multiplied by a reference sinusoid at an appropriate frequency. This method has proven to be inherently more efficient then rectifying to direct current.
When used to power a discrete system, however, the system taught in the York patent entails a one-to-one correspondence between the primary chopping phase and the secondary chopping phase. This one-to-one correspondence prevents the use of a single chopper with several secondary choppers for industrial applications that use more than a single output power type. There is, therefore, an unmet need in the art for an efficient means of conditioning the output of a generator allowing a one-to-many relationship between the primary chopper phase and the secondary chopper phases.
A direct conversion programmable power controller receiving polyphase input power at an input power frequency and with a polarity is provided. An exemplary embodiment of the controller includes a primary chopper for reversing each phase of electrical power according to a reference frequency that is substantially higher than frequency of the electrical power. Each primary chopper has a power input, a signal input, and an output. The power input is electrically connected in wye-connection to each phase of power. A transformer for each phase of power has primary and secondary terminals and is electrically connected by its primary terminals to the output of the primary chopper. A secondary chopper for each phase of power has an input electrically connected to the secondary terminals of the transformer. The secondary chopper is configured to reverse each phase of electrical power according to the reference frequency and shifts phases of electrical power according to a reference signal.
The invention modulates the received input power by rapidly reversing its polarity at a frequency significantly higher then the frequency of the input power. Because the modulating frequency is significantly higher than the input frequency, the resulting waveform is a substantially square wave over any of several wavelengths of the modulating waveform. Because the resulting modulated waveform integrates to zero over a period, use of the substantially square wave avoids saturation of the isolation transformer. Additionally, because of the suitably high frequency, the transformers used for isolation can be much smaller than those used for conveying power at a grid frequency from 50 to 60 Hz.
The invention also allows more then one secondary chopper to be connected to the output of a single primary chopper, thereby allowing distribution of the output of the primary chopper to be distributed on a power bus for secondary chopping at the application.